1. Field of the Invention
The invention relates to the field of supporting EFG crystal growth crucibles which are maintained at very high temperatures in the vicinity of about 2000.degree. C. or higher during the growth of crystals therefrom.
2. Prior Art
Crucible support systems which comprise a quartz tube supporting a graphite susceptor which in turn supports the crucible are known in the art for supporting EFG (Edge-defined Film-fed Growth) crucibles which are maintained at high temperatures, such as those in the neighborhood of 1200.degree. C. Unfortunately, such crucible support systems are not feasible for use at temperatures higher than about 1300.degree. C. because the softening or annealing temperature (1140.degree. C.) of the quartz is exceeded to an extent that the quartz deforms enough that it no longer provides a sufficiently stable support.
Prior art support systems for EFG crucibles which are maintained at very high temperatures for the growth of very high melting point crystals such as sapphire (melting point 2054.degree. C.) have utilized rods of refractory metals such as tungsten to support the crucible. Such support systems are inadequate for the highest quality crystal growth of wide ribbons because even refractory metals soften enough when exposed to these very high crystal growth temperatures that they bend excessively. Further, refractory metals have substantial coefficients of thermal expansion (tungsten's is 5.5.times.10.sup.-6 in/in/degree C.). As the temperature of the support rods increases, its expansion causes the position of the crucible and thus the position of the die top to change enough to make the control of the crystal growth even more difficult than it already is. Because of the large thermal gradients present in an EFG growth furnace, even a small change in the position of the die top within the furnace thermal profile can have a drastic effect on the crystal growth conditions at the liquid-solid interface where the crystal growth takes place.